1. Field of the Invention
This invention relates to a catalyst for a polyolefin system and a method of preparing an olefin polymer using such catalyst.
2. Description of the Prior Art
The catalytic production of polyolefins, such as polyethylene, is well known. It is already known to use an inorganic or organic magnesium compound with a transition metal compound as a high activity catalyst.
In an attempt to prepare polymer particles having a low amount of fine particles, it has been suggested in Japanese application 59-118120 filed Jun. 11, 1984, published as application 60-262802 on Dec. 26, 1985, to utilize a catalyst system that includes a mixture of an ingredient obtained by the reaction of magnesium, titanium, organoaluminum, silicon and halogenated aluminum compounds in sequence, and a catalyst ingredient which is an organometallic compound.
More particularly, it is disclosed in the Japanese application that a solid catalyst ingredient A is prepared by having a homogeneous solution of the combination of a metallic magnesium and a hydroxylated organic compound (or an oxygen-containing organic compound of magnesium) and an oxygen-containing organic compound of titanium react in succession with at least one kind of organoaluminum compound, then at least one kind of silicon compound and then at least one kind of halogenated aluminum compound. Catalyst ingredient A is mixed with an ingredient B which is at least one kind of an organometallic compound containing a metal from Group Ia, IIa, IIb, IIIa or IVa of the Periodic Table.
The reduction of the amount of fine particles of the polyolefin polymer is desirable for a number of reasons. The formation of deposits is inhibited during the polymerization reaction and during the processes of separation and drying of the polymer. Also, the scattering of fine particles of polymer outside of the system is prevented. In addition, separation and filtration of the polymer slurry is much easier because of the narrow particle size distribution, and the drying efficiency is enhanced due to the improvement in fluidity. Furthermore. during transportation of the polymer, bridging does not occur in the conduits or silos and problems with transferring the polymer are reduced.
Further, when the polymer is made by a multistage polymerization method, if the polymer has a wide particle size distribution, classification of the powder may occur in the reactor prior to transfer when the additive package is introduced and during the transportation stage after drying. Also, the quality of the polymer may be adversely affected since the physical properties typically are different for different particle diameters.
In addition, it is desirable to provide a polymer which has a narrow molecular weight distribution, as described in the Japanese application. This results in a polymer that has high impact strength.
Although it is stated in the Japanese application that excellent powder characteristics are obtained using the described type of catalyst system, it has been found that such catalysts still produce excessive amounts of polymer particles having a diameter less than 210 microns (referred to as fines) when used in a slurry reactor with an isobutane solvent.
Commonly assigned U.S. patent application Ser. No. 422,469, filed Oct. 17, 1989, and now U.S. Pat. No. 5,037,910, of the present inventors discloses an improvement over the type of catalyst and polymer process disclosed in the Japanese application. In such U.S. application, it is disclosed that the presence of a polyhydroxylated organic compound, in addition to the hydroxylated organic compound, significantly improves the particle size of the resultant olefin polymer and reduces the amount of fines. This is achieved while retaining the desirable properties of narrow molecular weight distribution and high catalyst activity
In the aforementioned U.S. patent application (the disclosure of which is herein expressly incorporated by reference), it is also disclosed that the addition of the halogenated aluminum compounds should take place at a temperature no higher than about 25.degree. C. in order to reduce the amount of fines.
Although it has been found that the addition of the polyhydroxylated organic compound is highly effective in increasing the polymer particle size and reducing the fines, nevertheless it would be desirable to further improve such catalyst in order to provide further enhancement in the properties of the resultant olefin polymer. In particular, it would be desirable to provide an improved catalyst system which increases the particle size and decreases the amount of fines, while retaining the desirable narrow molecular weight distribution and higher catalyst activity.